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5.2.1 Water temperature Caulerpa taxifolia in Moreton Bay Anecdotal evidence from professional fishermen has suggested that the distribution of C. taxifolia in Moreton Bay varies inter-annually (G. Savige, pers.comm.). A possible cause of this temporal variability could be changes of water temperature interseasonally. Surface water temperature data for Moreton Bay shows that the previous two winters (2001 and 2002) were warmer than preceding years (Figure 25; Ecosystem Health Monitoring Program, unpub. data). Temperature is one of the most important abiotic factors affecting macroalgal growth (Graham and Wilcox, 2000). C. taxifolia in the Mediterranean Sea displays high biomass and density in summer with some die-off evident over winter (Meinesz et al., 1995). Sustained high seawater temperatures have been associated with high population abundance of Caulerpa sertularioides (S.G. Gmelin) M. Howe in Mexico (Scrosati, 2001). Moreton Bay experiences similar minimum winter temperatures to the Mediterranean Sea, and these two populations of C. taxifolia exhibit similar cold tolerance thresholds (Komatsu et al., 1997; Chisholm et al., 2000; Phillips and Price, 2002). The 1998 and 2003 benthic surveys were conducted at similar times of the year (over summer), and the warmer winters preceding the 2003 surveys combined with the cooler winters preceding the 1998 surveys may have resulted in less die-off of C. taxifolia in winter. This may be partially responsible for the overall increase in C. taxifolia distribution observed since 1998 in Moreton Bay. Surface water temperature (° C) 30 25 20 15 10 Jan-96 Cool winters Jul-96 Jan-97 Jul-97 1998 surveys Jan-98 Jul-98 Jan-99 Jul-99 Jane Thomas -39- Discussion Jan-00 Jul-00 Jan-01 Warm winters Time Figure 25. Surface water temperatures of Moreton Bay from January 1996 to May 2003 (Ecosystem Health Monitoring Program, unpub. data). Values are means ± one standard error of the mean. Jul-01 Jan-02 Jul-02 2003 surveys Jan-03
Caulerpa taxifolia in Moreton Bay 5.2.2 Caulerpa taxifolia and seagrass interactions 5.2.2.1 Water quality The significant increase of C. taxifolia cover and spatial distribution in western and southern areas of Moreton Bay suggest that C. taxifolia distribution may be linked to Moreton Bay water quality. There is an east-west and a north-south gradient in water quality across Moreton Bay, with eastern and northern areas having generally better water quality as they receive greater oceanic flushing and less nutrient and sediment inputs (Dennison and Abal, 1999). In contrast, western and southern Moreton Bay are more impacted by human pressures, with sediment, stormwater and sewage inputs from adjacent rivers resulting in lower and more variable light penetration in these regions (Dennison and Abal, 1999; Longstaff, 2003). These gradients may help to explain localised changes in C. taxifolia distribution across Moreton Bay. In Moreton Bay, low water quality affects seagrasses primarily by light attenuation in the water column, principally from suspended sediment and, to a lesser extent, phytoplankton (Longstaff, 2003). This reduces not only the quantity but also the quality of light reaching seagrass (Dawes, 1998). Macroalgae, including C. taxifolia are typically tolerant of lower light conditions than are seagrasses which generally require up to 10 times more light than macroalgae (Lüning, 1990; Duarte, 1991; Dennison et al., 1993). Thus a decline in water quality in Moreton Bay may give C. taxifolia a competitive advantage, even though it is a native species. Alternatively, the opportunistic C. taxifolia may colonise the niche left vacant from seagrass loss due to poor water quality. Studies in the Mediterranean have associated C. taxifolia proliferation with urban wastewater discharge and seagrass loss, with C. taxifolia able to utilise the decaying seagrass vegetation in the sediment as a nutrient source (Chisholm et al., 1997; Chisholm and Moulin, 2003). C. taxifolia has significantly increased its distribution in southern and western Moreton Bay. In southern Moreton Bay it has replaced seagrass, while in western Moreton Bay it has mainly colonised bare sediment. Turbidity in southern Moreton Bay was at a minimum in 1998 but has been increasing since then (Qld Environmental Protection Agency, unpub. data) which may have contributed to the seagrass loss and C. taxifolia increase in this region. At the sites where C. taxifolia Jane Thomas -40- Discussion
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